This invention relates to a system and method for determining the geolocation of a transmitter, such as a mobile transmitting unit, and more particularly, this invention relates to a system and method of determining the geolocation of a mobile transmitting unit using reflectors/refractors.
In prior art location determining systems where Time of Arrival (TOA) and/or Angle of Arrival (AOA) methods are used, multipath is a significant contributor to errors. For example, one global positioning system (GPS) based location system has a reported Root Mean Square (RMS) error of 7 meters when the GPS receiver is in a suburban sidewalk or clear area. In the more congested street canyons between high-rise buildings, however, the reported Root Mean Square error is approximately an order of magnitude greater. These errors arise from the inability of the receiver to determine times of arrival (either absolutely or comparatively) of signals propagating via straight-line paths from the satellites to the receiver in the multipath environment. In many instances, the energy in a straight-line path is not even detectable.
Some mobile phone locating systems use a network of receivers to measure the time of arrival (TOA) or the angle of arrival (AOA) of the phone signal at plural receive sites and use these measurements in a multilateration or triangulation process to determine the location of the phone. These systems require that three receivers measure the signal Time of Arrival or Angle of Arrival in order to resolve ambiguities in the location estimate. The accuracy of these cell-phone overlay systems is also degraded by multipath. Errors are introduced by the inability of the receiver to detect and measure the time of arrival or angle of arrival of that portion of the signal propagating in a straight-line path from the transmitter to the receiver. In addition, those systems that employ time-difference-of-arrival (TDOA) calculations are also subject to errors introduced by errors in the time references at the plural receive sites.
Other location systems use combined Angle of Arrival and Time Difference of Arrival (or a similar technique) where the Time of Arrival and the Angle of Arrival of the signal are measured at two or more receive sites. An advantage of these systems relative to those that use only Angle of Arrival or Time Difference of Arrival is that the location of the transmitting unit may be unambiguously determined from data from two receive sites.
One example of such a location system is shown in U.S. Pat. No. 5,719,584 to Otto, assigned to the present assignee, Harris Corporation of Melbourne, Fla., the disclosure of which is hereby incorporated by reference in its entirety. In the ""584 Otto patent, a system and method determines the geolocation of a transmitter within or without a set of receiving stations. Plural receiving stations determine the Time of Arrival and Angle of arrival of a signal from a transmitting unit, such as a mobile transmitting unit. A central processing unit determines the geolocation of the radiating unit from such Times of Arrival and Angles of Arrival data. If the measured Angle of Arrival and Time of Arrival are not that of the straight-line path, errors in the location estimate will result. This system is also subject to errors in TOA measurements caused by inaccuracies in the time references at the receive sites.
Other patents, such as U.S. Pat. No. 6,184,829, assigned to True Position, Inc., concerns a method and apparatus for calibrating a wireless location system to make highly accurate Time Difference of Arrival and Frequency Difference of Arrival measurements. A first reference signal is transmitted from a reference transmitter and received at first and second receiver systems. A first error value is compared with a measured Time Difference of Arrival or Frequency Difference of Arrival value with a theoretical time difference of arrival or a frequency difference of arrival value associated with the known locations of the receiver systems and the known locations of the reference transmitter. The first error value is used to correct subsequent Time Difference of Arrival measurements associated with the mobile transmitter to be located. An in internal calibration method injects a comb signal into the first receiver system. The comb signal is used to obtain an estimate of the manner in which the transfer function varies across the bandwidth of the first receiver. This mitigates the variation of the first transfer function on the time measurements made by the first receiver.
Another prior art location system incorporates a radio frequency xe2x80x9cfingerprintingxe2x80x9d technique in an attempt to take advantage of multipath in the location determining process. In this system a database of received signal signatures or xe2x80x9cfingerprintsxe2x80x9d is generated during a calibration process for each receiver in the system. Each calibration fingerprint consists of a set of signal parameters measured by that receiver when a transmitter is transmitting from a known location or grid point. The database consists of a large set of these fingerprints with each one referenced to its corresponding grid point. In this location system, the radio frequency fingerprint of the mobile transmitting unit to be located is measured, and the location estimate is the grid point associated with a xe2x80x9cmatchingxe2x80x9d fingerprint from the database. By using this technique, it is possible under the appropriate circumstances to locate the mobile transmitter in a high multipath environment with a single receiver.
This type of location system has several drawbacks and limitations. For example, a mobile transmitting unit is presumed to be at one of the grid points, instead of intermediate of various grid points. Thus there is an inherent location estimate xe2x80x9cquantizationxe2x80x9d error related to the density of the grid points. Generation of the fingerprint database, often referred to as xe2x80x9ccalibrationxe2x80x9d, requires the costly and time-consuming process of having a transmitter travel to and transmit from each grid point. Once the calibration process is complete, changes in the local skyline caused by the erection or tearing down of a building or other structure result in changes to the fingerprints for grid points in the vicinity and, therefore, re-calibration is required.
Relocation of a receiver also requires that the calibration process be repeated for that receiver. Typically, the grid points are at street level due to the difficulty of getting access to every building or structure during the calibration process and due to the increase in cost associated with generating a three dimensional grid as opposed to a two dimensional grid. It is well known by those skilled in the art, however, that the multipath profile, or fingerprint, of a transmitter changes dramatically in an urban environment as the transmitter goes up in elevation. Thus, a transmitter carried by an individual on the 30th floor of a building would have a fingerprint that would vary dramatically from that generated by a transmitter located on or near the ground floor of the same building during calibration. The differences may be so great that the fingerprint does not match the fingerprint for the correct grid point or, perhaps even worse, the fingerprint matches the fingerprint of another grid point some distance removed.
The fingerprint of a transmitter attached to or embedded in an object such as an asset may also vary dramatically from the fingerprint of the transmitter used for calibration due to shadowing or directional blockage induced by the object. This may result in failure to find a matching fingerprint in the database or result in a match to a fingerprint for a grid point far removed from the actual location of the transmitter. The use of the fingerprinting technique is not readily applicable to mobile receivers since calibration would have to be repeated for every possible location of the mobile receiver. The expense associated with generating a database containing fingerprints for every grid point at each of the possible locations of the mobile receiver is prohibitive.
It is therefore an object of the present invention to provide a system and method for determining the location of a transmitter that is operable with a database that is not necessarily extensive and that may be readily modified or enhanced with minimal expense.
In accordance with the present invention, the system determines the location of a transmitter and includes a transmitter to be located that transmits a signal. At least one receiver receives the signal from the transmitter. At least one proxy receiver is an object that reflects and/or refracts a signal along an arriving path to the at least one receiver. A processor is operative with the at least one proxy receiver and determines the location of the at least one proxy receiver based on one of querying a look-up table or extracting image features from a mapping system and determining the location of the transmitter based on the time of arrival at least one of angle of arrival of signals from the at least one proxy receiver transmitter.
The at least one proxy receiver can include two proxy receivers. The system further comprises a graphics application program and application programming interface, and a three-dimensional model of a geographic area for determining the location of proxy receivers based on signals received along the arriving path to the receiver. The system further includes a site model map image containing composite maps in a spatially correct position. These composite maps can include texture maps.